1. Field of the Invention
The present invention relates to a boost-up power factor correcting circuit using a power feedback, and more particularly to a boost-up power factor correcting circuit using a power feedback which can correct a power factor by a power feedback in high frequency current using an auxiliary winding to be wound in an SMPS transformer.
2. Description of the Related Art
In general, industrial or household electric appliances use direct current (DC) as their operating power sources. The DC power sources are generally produced by a circuit rectifying commercial alternating current (AC) power sources, such as a capacitor-input type rectifying circuit which has a simple circuit construction. However, the capacitor-input type rectifying circuit has a low power factor because the input current has a pulse type of flowing only over the peak part of the input AC voltage. For this reason, the electric appliances generally employ a power factor correction circuit.
FIG. 1 shows a power supply circuit of an electric appliance employing a conventional power factor correcting circuit. Referring to FIG. 1, the power supply circuit of an electric appliance employing a conventional power factor correcting circuit comprises a rectifying section 70 for full-wave rectifying the inputted AC voltage, a power factor correcting section 71 for correcting a power factor of the power source rectified by the rectifying section 70, an SMPS 72 for switching so as to convert the output voltage of the power factor correcting section 71 into a predetermined constant voltage, a microcomputer 73 for comprehensively controlling an operation of the image displaying appliance including the SMPS 72.
The power factor correcting section 71 includes an inductor L connected to the rectifying section 70 for accumulating a magnetic energy, a diode D connected between the inductor L and the SMPS 72, an FET 74 connected between the inductor L and the anode of the diode D for performing a high-speed switching operation, an FET control IC 75 connected to the gate of the FET 74 for controlling the switching operation of the FET 74. A capacitor C is connected between the cathode of the diode D and the drain of the FET 74.
The operation of the conventional power factor correcting circuit constructed above will now be described with reference to FIG. 1.
If an alternating current (AC) power source is inputted to the rectifying section 70, the rectifying section 70 full-wave rectifies the inputted AC power source and outputs it to the inductor L of the power factor correcting section 71. Then, the rectified power source is inputted to the capacitor C through the inductor L and the diode D. Thus, the capacitor C smooths the rectified power source and outputs it to the SMPS 72. The SMPS 72 voltage-converts the inputted DC power source inputted according to the control signal of the microcomputer 73 into a constant voltage required by each part of the image displaying appliance.
At this time, the FET control IC 75 of the power factor correcting section 71 switches the FET 75 at high speed so as to correct the power factor of the DC power source applied to the SMPS 72.
That is, if the FET control IC 75 applies the switching control signal to the gate of the FET 74, the FET 74 is turned on. Consequently, the inductor L cannot input the rectified voltage inputted from the rectifying section 70 to the diode D. However, if the FET control IC 75 does not apply the switching control signal to the FET 74, the FET 74 is turned off. Then, the inductor L outputs the accumulated magnetic energy to the capacitor C through the diode D. Since the operation described above is performed at high frequency current by the FET 74, the power factor of the charged current is inputted from the capacitor C to the SMPS 72 after being corrected.
However, such a conventional power factor correcting circuit has a disadvantage in that the manufacturing cost of the power factor correcting circuit is notably increased due to an employment of the FET 74 of high cost. Further, the circuit is complicated since a separate driving switching element, such as the FET control IC (75), is used for the FET 74.